Hello lazar,
Paizo Advantage becomes active when each one of your four subscriptions has shipped their first item. This means that with your current arrangement, your discount would not become active until the release of the Gamemastery Guide. In order to have the discount become active immediately, you would have to begin this subscription with an item that is already available to ship, AKA the Core Rulebook and Bestiary. This is what is preventing the discount from becoming active. If you need to make any changes to your subscriptions with this clarification, please let us know. Thank you!